A permanent magnet machine is a type of alternating current (AC) motor whereby power is supplied to the rotating device by means of imposing an electromagnetic field onto a field generated by a permanent magnet rotor. To control an AC permanent magnet machine, the absolute position of the magnetic field produced by the rotor permanent magnets (PM) needs to be known at all times. The magnet's position is important for determining the stator current magnitude of the machine necessary to produce the desired torque. This operation is known as the stator current commutation. Since magnets are rotating with respect to the stator, the magnet position can be calculated by measuring the rotor absolute angular position. Absolute position sensors such as resolvers and optical encoders are typically used to measure the absolute angular position of the machine rotor.
H-bridge topologies are commonly used to drive open delta permanent magnet machines, as well as other types of machines. An H-bridge is an electronic circuit which enables a voltage to be applied across a load in either direction. The term “H-bridge” is derived from the typical graphical representation of such a circuit as seen in FIG. 1A. An H-bridge is built with four switches, which may be solid-state or mechanical. As seen in FIG. 1B, when switches S1 and S4 are closed, and switches S2 and S3 are open, a positive voltage will be applied across the motor. However, by opening switches S1 and S4, and closing switches S2 and S3 (FIG. 1C), this voltage is reversed allowing reverse operation of the motor. An H-bridge arrangement may be used to reverse the polarity of the motor, but may also be used to stop the motor if the motor's terminals are shorted.
One of the main benefits for using an H-bridge to drive a PM machine is that it lowers the ripple current in the motor windings that results from the pulsating voltage waveforms of the pulse modulated power stage. Ripple current is a small unwanted residual periodic variation of the direct current output of a power supply which has been derived from an alternating power source. Another benefit garnered from driving a PM machine with an H-bridge is that the H-bridge allows a greater level of flexibility to shape the current waveform beyond the 120-degree limitation imposed by a conventional 3-phase inverter.
To properly control the currents and to minimize the harmonic content in the current waveforms, robust control must be performed. One of the issues with H-bridge control is that in order to reduce the machine ripple current, unipolar pulse width modulation (PWM) must be employed. With unipolar PWM, a 50% reduction in ripple current is achieved when compared to bipolar PWM. When using unipolar PWM during transition periods where the machine emf voltage has a different polarity from the terminal voltage, a combination of unipolar and bipolar PWM must be employed; otherwise, current distortion will result. The existing prior art merely selects unipolar verses bipolar PWM based upon the sign of the current waveform. This conventional approach leads to additional harmonics in the current waveform that will lead to increased heating in the machine.